April 17, 2009—Save the males? Too late for Mycocepurus smithii (pictured).

This leaf-cutter ant species is all female and thrives without sex of any kind—ever—according to a new study. The ants have evolved to reproduce only when queens clone themselves.

"They appear to have evolved a new mode of reproduction, and the genetic mechanisms have yet to be worked out," said lead study author Anna Himler, a research associate at the University of Arizona.

In M. smithii the typical muscular reproductive organ of female ants has evolved into a "sort of a ghost of an organ at this point," Himler added.

No male of the species has ever been found, and "even if a male were theoretically to appear somewhere, we're not sure they could mate any more," she said.

Other ants, such as fire ants, that can reproduce asexually have working sexual organs, just in case.

M. smithii also has an idiosyncratic arrangement for that other apparent necessity: food. The ants, which range from northern Mexico to Argentina, are in a codependent relationship with a specific fungus.

"The fungus garden is quite wimpy," Himler said. "If you remove the ants, the gardens will quickly die."

The ants keep the gardens weeded, and they "feed" the fungus leaf bits, insect carcasses, and feces, which the ants clean and cut up before offering to the fungus. In return, the fungus provides the sole source of food for the ants' babies.

—Anne Minard

Findings appeared online April 15 in the journal Proceedings of the Royal Society.